US President-elect Donald Trump has posted a series of tweets criticising China for its monetary policy and its operations in the South China Sea.
“Did China ask us if it was OK to devalue their currency” and “build a massive military complex?” he asked. “I don’t think so!”
Last week Mr Trump risked a diplomatic rift with China by speaking directly with Taiwan’s president.
The highly unusual move saw China lodge a complaint with the US.
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The US has previously criticised China’s yuan devaluation, saying it unfairly favours Chinese exporters.
It has also told Beijing to stop reclaiming land around islands and reefs which are claimed by multiple countries in the South China Sea, and has sent US Navy ships to the area. Both sides have accused each other of “militarising” the region.
China has said its land reclamation efforts are for civilian purposes.
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The US currently imposes tariffs on some Chinese imports, such as steel and tyres. Mr Trump has previously threatened to impose a 45% tariff on Chinese goods.
Mr Trump’s phone call with Taiwan’s President Tsai Ing-wen was thought to be the first time a US leader or leader-in waiting has spoken to a Taiwanese leader since 1979, the year formal ties were severed.
The White House has said the phone call did not signal a shift in its decades-long “One China” policy stance, which considers Taiwan to be part of China.
Vice President-elect Mike Pence has tried to downplay the call.
In an interview with NBC News on Sunday, he said it was a “tempest in a teapot” and added: “I think I would just say to our counterparts in China that this was a moment of courtesy.”
Beijing lodged a “solemn representation” with Washington, where it urged the US to “cautiously and properly handle” the issue of Taiwan, according to Chinese state media.
Taiwan sees itself as an independent state but Beijing considers it as a breakaway province.
It has hundreds of missiles pointing towards Taiwan, and has threatened to use force if Taiwan formally declares independence.